The lack of rigorously-validated outcome measures remains a substantial hurdle to the performance of excellent clinical trials in many forms of systemic vasculitis. In this proposal, leaders of the International Network for the Study of the Systemic Vasculitides (INSSYS) and the Vasculitis Clinical Research Consortium (VCRC) will build upon the productive Collaborations of their combined Research Groups. The investigators will also leverage the resources provided by funded research initiatives spearheaded by the Co-P.I.s, Drs. John Stone and Peter Merkel. There are two Specific Aims: Aim 1: To develop and validate a consensus disease activity measure for ANCA-associated vasculitis. This new consensus outcome measure ("CANVAS") will be validated in clinical trials slated to begin in both the United States and Europe. The results of this Aim will also provide a basis for defining three critical states of disease activity: remission, flare, and stable/persistent disease. These definitions are crucial to identifying such central clinical trials outcomes as partial responses and complete remissions. Aim 2: To develop magnetic resonance (MR) imaging and positron emission tomography (PET) as clinical trial outcome measures in large vessel vasculitis. This Aim will be achieved through data-driven assessments of patients involving a collaboration of rheumatologists, radiologists, statisticians, and bioinformaticists. The project capitalizes upon advances in these complementary imaging modalities that provide information about both narrowing of the vessel lumen and the state of biological inflammation within the blood vessel wall. Both projects involve leaders in vasculitis research and clinical trials from the United States and Europe. The clinical vasculitis investigators are supplemented by a group of experts who have extensive experience in outcome measures development and possess other pertinent skills.